The field of the invention pertains to sensors wherein changes in the physical size of a magnetic circuit are driven by or sensed by electric coils through which the magnetic circuit passes. Typically, such a device comprises an exciting coil and a sensing coil joined by a magnetic circuit of high permeability combined with physical stress/strain characteristics that measurably affect the physical dimensions of the magnetic circuit and the magnetic permeability.
Magnetostrictive sensor structures are disclosed in applicant""s previous U.S. Pat. No. 5,437,197 wherein an magnetic circuit passes through two coils. One coil excites the magnetic circuit by means of an alternating electric current and the second coil senses changes in magnetic flux as changes in physical forces applied to the magnetically permeable circuit occur. In alternative embodiments dynamic sensors comprise a direct current applied to the exciting coil or the substitution of a permanent magnet for the exciting coil to produce a constant exciting flux in the magnetic circuit.
U.S. Pat. No. 5,297,439 discloses a sensor comprising a magnetostrictive material inductively coupled to a simple electric resonance circuit. Changes in strain of the magnetostrictive material cause changes in the resonance frequency of the electrical signal which, in turn are picked up by a nearby aerial and transmitter/receiver.
U.S. Pat. No. 4,955,241 discloses a magnetoelastic force measuring device comprising a soft magnetic measuring film in a configuration that provides good thermal error compensation. In another manner U.S. Pat. No. 5,007,295 discloses a magnetoelastic force transducer configured to compensate for non-force induced changes in the magnetic permeability of the device. U.S. Pat. No. 4,823,621 discloses a magnetoelastic force transducer having similar compensation means as the two patents above but with an asymmetric center pole configured to compensate for any measurement signal arising under zero applied force on the device. U.S. Pat. Nos. 4,802,368 and 4,825,709 both disclose configurations to compensate for temperature changes and other non-force induced changes in the magnetic permeability of a thin walled component of the magnetic circuit.
Also, of background interest are three papers co-authored by applicant and published by the American Institute of Physics. A Noncontacting Magnetostrictive Strain Sensor, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 64(8), August 1993, discloses a sensor wherein the portion of the magnetic circuit subject to strain is separated from the rest of the magnetic circuit by air gaps.
A magnetostrictive Force Sensor, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 65(5), May 1994, discloses a sensor wherein the cores of the exciting coil and sensing coil are the principal magnetostrictive elements of the permeable circuit. Modelling of Magnetostrictive Sensors, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 67(1), January 1996, discusses the mathematical modelling of the sensors disclosed in the above two papers.
With a view toward simplifying the above sensors and, therefore, providing more economically constructed sensors for mass production and installation, the applicant has developed the following improved sensors.
The invention comprises a magnetostrictive force or strain sensor having one electric coil to both excite the magnetostrictive circuit and detect changes in the permeability of the circuit arising from forces applied to the circuit or induced to strain a portion of the circuit. The new sensor may be constructed to sense either or both compressive and tensile forces applied to the magnetostrictive circuit. Thin foils or plating applied to non-magnetically permeable materials may comprise part or all of the magnetostrictive circuit to provide an exceptionally inexpensive and light weight structure for the sensor. A permanent magnet may be incorporated into the structure to provide a sensor capable of measuring dynamic forces, especially those of impact, explosion and relatively high frequency.
The new sensors have a wide range of applications including but not limited to occupant sensing and weighing in automobile airbag systems, engine, powertrain and suspension controls, and monitoring highway bridge loads.